So, here is what's happening:
I create a new document. I set the size to US Letter, units to mm, and set my margins to: top 60mm. bottom 6 mm. left 15mm and right 15 mm.
I look at the output on the screen, and the margin on the left looks slightly smaller that the one on the right. Thinking it's an optical illusion, I draw a box the exact size of the margins, and then save it to PDF. I print out the PDF and measure the box. The left side is 14 mm from the edge of the page and right side is 17 mm from the edge of the page.
I print on a different printer and I get the same results, 1 mm short on the left, and 2 mm over on the right.
This is happening on two different Macs. I have not had a chance to try Windows or Linux yet.